I havent done the gauntlet yet myself so I couldnt comment on the damage but I agree that 3 healers is possibly too much. The shaman was an officer so perhaps he merely wanted to oversee things.

3 is too many. Two healing is fairly easy even without anything resembling control, and with a little coordination solo healing it should also be entirely doable.

As another already said, it seems you get the best value out of using your entire inventory of spells in Ulduar. For me before Ulduar it was something of a rotation of PoM, FH and CoH, with more FHs than anything. In Ulduar the use of PoH has gone up greatly. Divine Hymn has been nothing short of awesome. Using inner focus to cast it, as in setting up a macro for that use is helpful.

I think Ulduar and the change to PoH makes haste and intelligent use of Serendipity more valuable. On a couple fights I swap in some haste gear to add more haste and change my food buff to add an additional 40 haste. These fights are XT-002 and Ignis.

Build up Serendipity in anticipation of casting PoH for your best results

The more liberal use of GS that others spoke to also seems beneficial.

As Priests it seems we can do well in Ulduar, especially once we have seen the encounters a few times and can know what to expect and prepare for.

Our setup for Thorim is 12 in the Arena (3 healers, 2 tanks, 7 dps) and 13 in the Tunnel (4 healers, 2 tanks, 7 dps). We could easily drop to 3 healers in the tunnel, but it's not worth it for the stability we gain in Phase 2. Our typical split-up is paladin/shaman+priest+druid in the Arena, and paladin+shaman+priest+shaman/druid in the Tunnel. It's really not that hard to heal; it's mostly on the dps to avoid being gibbed by WW and lightning balls.

We go with 2 healers, 1 tank and 6 dps in the tunnel. Works out well, we cut some corners so we're still able to complete it with just a few seconds from activating hard mode.

Arena is more about dps than healing. It's very helpful to have a dps shaman in the arena to purge the shields as well as interrupting heals. We use 4 healers outside, 1 druid, 2 holy priests and whatever. As soon as we got the dps part correct, stopped the healing from adds and tanked the WW adds outside of range, I'm sure we could have done it with even less healers. We tried stacking 5 healers outside at first, but it didn't help anything. It's easy to blame healers when you start getting overrun and people die, but it's all about control and tanking. We use 2 tanks, 2-3 healers and 5-6 dps in the gauntlet usually, and we usually just miss hard-mode. Since we don't want to try hardmode just yet, that doesn't matter.

I was surprised when you placed such importance on lightwell and was very curious as to how much healing it did over your entire run.

If the WWS is correct it did 15,000 healing. To you. Just to be sure I checked all the melee's healing and they didn't record any healing done using lightwell. Maybe WWS is broken in this regard.

Either way, with exception to general I don't want our DPS distracted by lightwell. Maybe if the healing didn't die after taking damage it'd be better.

The HoT cancels itself when you take a hit for more than 30% of your HP (since 2.4 or so...?), not when taking damage anymore.
'Usual' Raid damage is low enough not to break it, I guess.

Lightwell

It doesn't get killed by Frozen Blows? I thought random AOEs used to blow it up.

Best strategy we found for Thorim is this:
1 tank (Feral-druid), 2 healers (disc-priest + resto-druid) and 7 DPS (4mages).
First group: acolytes get sheeped, guardian killed.
Second group: acolytes get sheeped, guardian killed.
Runic Colossus is pulled while acolytes are being killed to stop the annoying fires.

All adds on stairs are sheeped and the rune-giant is pulled and killed.
Then the adds on stairs are killed untill it's safe to pull Thorim for 'easy' mode. The remaining adds (if any) are picked up in the arena and killed there.

This way makes the gauntlett very easy and without risks and enables us to go for 'hard-mode' if we want without changing anything.

In the arena we have 3 tanks, 4 healers -one of those (priest) is basically mass-dispelling fulltime to remove the shields- and 8 DPS.

2 healers and 4 in the arena is all that is required. Ensuring in the gauntlet you have one tank healer & one good at ranged.